The output powers, efficiencies, and lifetimes of short-wavelength ultraviolet light-emitting diodes (UV LEDs), i.e., LEDs that emit light at wavelengths less than 350 nm, based on the nitride semiconductor system remain limited due to high defect levels in the active region. These limitations are particularly problematic (and notable) in devices designed to emit at wavelengths less than 270 nm. Most development effort has been carried out on devices formed on foreign substrates such as sapphire where defect densities remain high despite innovative defect reduction strategies. These high defect densities limit both the efficiency and the reliability of devices grown on such substrates.
The recent introduction of low-defect, crystalline aluminum nitride (AlN) substrates has the potential to dramatically improve nitride-based optoelectronic semiconductor devices, particularly those having high aluminum concentration, due to the benefits of having lower defects in the active regions of these devices. For example, UV LEDs pseudomorphically grown on AlN substrates have been demonstrated to have higher efficiencies, higher power and longer lifetimes compared to similar devices formed on other substrates. Generally, these pseudomorphic UV LEDs are mounted for packaging in a “flip-chip” configuration, where the light generated in the active region of the device is emitted through the AlN substrate, while the LED dies have their front surfaces bonded to a polycrystalline (ceramic) AlN submount. Because of the high crystalline perfection that is achievable in the active device region of such devices, internal efficiencies greater than 60% have been demonstrated. Unfortunately, the photon-extraction efficiency is often still very poor in these devices, ranging from about 4% to about 15% achieved using surface-patterning techniques.
For several reasons, the photon extraction efficiency from short-wavelength UV LEDs is poor compared to visible LEDs. Thus, the current generation of commercial short-wavelength UV LEDs has low wall-plug efficiencies (WPE) of, at best, only a few percent, where WPE is defined as the ratio of usable optical power (in this case, emitted UV light) achieved from the diode divided by the electrical power into the device, and the best reported “hero” devices have claimed WPE of just over 10%. The WPE of an LED can be calculated by taking the product of the electrical efficiency (ηel), the photon extraction efficiency (ηex), and the internal efficiency (IE); i.e., WPE=ηel×ηex×IE. The IE itself is the product of current injection efficiency (ηinj) and the internal quantum efficiency (IQE); i.e., IE=ηinj×IQE. Thus, a low ηex will deleteriously impact the WPE even after the IE has been improved via the reduction of internal crystalline defects enabled by, e.g., the use of the AlN substrates referenced above as platforms for the devices.
Several issues can contribute to low photon-extraction efficiency. First, even the highest-quality AlN substrates available generally have some absorption in the UV wavelength range, even at wavelengths longer than the band edge in AlN (which is approximately 210 nm). This absorption tends to result in some of the UV light generated in the active area of the device being absorbed in the substrate, hence diminishing the amount of light emitted from the substrate surface. Additionally, UV LEDs suffer because approximately half of the generated photons are directed toward the p-contact and absorbed by the p-GaN of that contact. Even when photons are directed toward the AlN surface, only 9.4% can escape from an untreated surface due to the large index of refraction of the AlN, which results in a small escape cone. Additional photons are lost on their way to the exit surface due to absorption in the AlN wafer. These losses are multiplicative and the average photon extraction efficiency is only about 2.5%.
Since photon absorption by the AlN substrate and the high refractive index contrast between air and AlN deleteriously impact the photon-extraction efficiency of UV LEDs on AlN, these effects may be ameliorated via removal of all or a portion of the substrate. Various techniques have been developed for removal of substrates in other materials systems, but such techniques are generally not effective when utilized for UV LEDs on AlN substrates. Moreover, encapsulation techniques utilizing rigid lenses have been utilized to enhance photon-extraction efficiency—see, e.g., U.S. Pat. No. 8,962,359, filed on Jul. 19, 2012, the entire disclosure of which is incorporated by reference herein—but such techniques generally do not address substrate absorption, nor do they address the issue of photons that are propagating at very small angles to the interface and which are therefore unlikely to escape from a thick substrate.
In view of the foregoing, there is a need for improved techniques for the removal of all or a portion of an AlN substrate of a UV light-emitting device.